This invention relates to a process for the manufacture of chemical absorbents, and to novel chemical absorbent formulations.
Chemical absorbents are used to remove one or more molecular species, for example to remove specific molecular species from mixtures of gases. Soda lime is one such chemical absorbent and is widely used to absorb carbon dioxide, for instance in anaesthetic breathing systems and other applications involving air which is to be breathed.
In order to make chemically active soda lime, calcium hydroxide is mixed with sodium hydroxide and/or potassium hydroxide to produce a caustic lime mixture containing water. The ratio of components in the caustic lime mixture is 96-98% calcium hydroxide with 2-4% sodium and/or potassium hydroxide. The finished product contains typically 81-87% caustic lime mixture and 13-19% water. In addition, a pH sensitive indicator dye is normally present to give a visual indication of the extent of usage and exhaustion. This indicator dye is present in the caustic lime mixture at very low levels, typically 0.01-0.1%.
To varying degrees, all known soda lime formulations contain dust and fine particles generated as a result of the friability and breakdown of the finished product. The presence of such dust is undesirable, particularly when the product is intended for use in anaesthetic (or other) breathing systems since the fine particles may be inhaled.
There has now been devised a process for the manufacture of chemical absorbents which overcomes or substantially mitigates the above problem.
According to the invention, a process for the manufacture of a chemical absorbent in solid, granular form comprises the steps of
a) mixing the components of the chemical absorbent with water to form a paste; and
b) passing the paste between a pair of counter-rotating, contacting rollers, the rollers having formed therein corresponding substantially hemispherical depressions.
The process according to the invention is advantageous primarily in that the chemical absorbent is formed into substantially spherical granules. Consequently, the granules have no projecting edges or corners which can break off and create dust. The granules are considerably less friable than conventional granules and maintain the integrity of their shape throughout normal use.
The surface and rotation of the counter-rotating rollers are such that, at the point at which the rollers touch, depressions in the rollers form substantially spherical moulds. Most preferably, the hemispherical depressions are arranged in a hexagonal close-packed array.
Preferably, the direction of rotation of the rollers and the direction in which the paste is fed between them are such that the paste travels downwards, ie movement of the paste between the rollers is assisted by gravity.
As the rollers rotate, the spherical moulds re-open, exposing the moulded paste spheres. It is preferred that means be provided for dislodging the spheres from the rollers. Most preferably, such means is an air knife positioned adjacent each roller to generate a tangential blade of high velocity and energetic air.
Spherical solid granules of a chemical absorbent formulation are believed to be novel, and represent a further aspect of the present invention.
The process according to the invention may be utilised to produce spherical granules of a wide range of chemical absorbent formulations. However, it has been found to be particularly useful in relation to soda lime formulations. Furthermore, novel soda lime formulations have been developed which demonstrate improved mechanical strength and are especially well suited to manufacture by the process of the invention.
Thus, according to another aspect of the invention there is provided a soda lime formulation comprising a major proportion of calcium hydroxide in admixture with a minor proportion of sodium hydroxide and/or potassium hydroxide and water, the formulation further comprising a zeolite.
Various forms of zeolite, eg zeolites containing sodium, calcium, barium, strontium or potassium, may be utilised. The presently preferred zeolite is sodium aluminium silicate.
The zeolite is preferably present in the formulation to a level of between 0.1 and 10% w/w, more preferably 2% to 6% w/w.